Imam Al-Nawawi’s Riyad-us-Saliheen
Undesirability of blowing into the Vessel while Drinking
765. Abu Sa`id Al-Khudri (May Allah be pleased with him) reported: Messenger of Allah (sallallaahu alayhi wa sallam) prohibited us blowing in the drinking water. A man said: “O Messenger of Allah! Sometimes I see some litter floating about on the surface. What should I do then?” He (sallallaahu alayhi wa sallam) replied, “Pour them out.” Then the man said: “My thirst is not quenched with one draught.” Messenger of Allah (sallallaahu alayhi wa sallam) said, “Then put away the cup from your mouth (in between three gulps), and take breath.”
766. Ibn `Abbas (May Allah be pleased with them) reported: The Prophet (sallallaahu alayhi wa sallam) prohibited us from breathing into the drinking vessel or blowing onto.